In backbone optical fiber communication transmission systems, the wavelength multiplexing optical transmission technology has been used. This technology discretely allocates signals to a plurality of different wavelengths over one transmission line fiber so as to realize a large capacity.
In this wavelength multiplexing transmission technology, the realization of narrow wavelength intervals for large capacity depends on the transmission speeds of signal lights allocated to individual wavelengths. Thus, the wavelength intervals are decided by signal speeds that are dominant when the system operations are started. For example, the current dominant 50 GHz interval and 100 GHz interval have been decided on the basis of the spectrum width of a 10 Gb/s (bit/second) signal light.
If higher speed signal lights, for example, 40 Gb/s and 100 Gb/s signals, are tried to be introduced for such a system in which a wavelength interval has been decided, various problems will arise. For example, because the spectrum of a high speed signal light widens, it does not fit the wavelength interval, resulting in the occurrence of a crosstalk to adjacent channels.
Thus, when a signal light that is faster than the speed that was estimated when the system operation was started (for example, 40 Gb/s or faster) is introduced, it is important to apply a modulation system that compresses the spectrum width and a data superimposing system to the existing system such that the signal light spectrum fits the wavelength interval of the existing wavelength multiplexing transmission system.
Among modulation systems, in particular, the polarization multiplexing and splitting system is a hopeful technology to compress the spectrum width. In the polarization multiplexing and splitting system, on the transmission end, two independent signal lights of the same wavelength are allocated to two polarization axes of an optical fiber to perform polarization multiplexing, whereas on the reception end after transmission, the multiplexed signal light is split again into two polarization components (polarization splitting) and they are individually received.
This polarization multiplexing and splitting system has been implemented for a long time and was used in an experiment reported in 1996 as the first 1-tera bits/fiber transmission experiment in the world as described in Non-Patent Document 1.
In the experiment described in Non-Patent Document 1, an objective of polarization multiplexing was to share one wavelength by two signal lights, namely improve frequency usage efficiency. In contrast, the current interest as to polarization multiplexing is in that one data sequence is split into two parts (for example, a data sequence of 100 Gb/s is split into two data sequences of 50 Gb/s) and the split data sequences are allocated to the individual polarization components so as to compress the spectrum width.
When one data sequence is split into two sequences, they are not independent, for example, the data speeds of signal lights allocated to two polarization components are not perfectly the same; however, instead, this characteristic in which the data speeds of these data sequences are the same is often positively used.
For example, as described in Non-Patent Document 2, in the pulse interleave polarization multiplexing in which two polarization components are individually RZ (return-to-zero) formatted and are allocated such that the peak positions of RZ pulses are shifted by a half bits therebetween, the characteristic of which the data speeds in both the polarization components are the same is used to decrease the occurrence of a linear/nonlinear crosstalk between both the polarization components.
In this circumstance, when polarization multiplexed signal lights were generated in the past, both polarization components were the same with respect to not only the data speed, but also the modulation and demodulation systems. In other words, both polarization components were symmetrical.
In the polarization control of reception side polarization splitting that requires the most advanced technique in the polarization multiplexing and splitting system, this symmetry has been positively used.
For example, in Patent Document 1, as the control rule of polarization splitting, a symbol speed component contained in one of the output components after polarization components is split in order to be maximized. This is also a technology that can be used only when both polarization components are symmetrical.